The term "cook-in" as used herein is intended to refer to packaging material structurally capable of withstanding exposure to time-temperature conditions while containing a food product.
Cook-in packaged foods are essentially foods cooked in the package in which they are distributed to the consumer or other purchaser, and which may be consumed with or without warming. (In some cases, the food product is removed from the original package after cooking, and repackaged in another packaging material for distribution.) Cook-in time-temperature conditions typically refer to a long slow cook, for example, heating the product to an internal temperature of between about 55.degree. C. and 75.degree. C. for between about 1 and 4 hours. These conditions are representative of institutional cooking requirements. Under such conditions, a cook-in packaging material should maintain its integrity, i.e., the product should maintain its interply adhesion, and any heat sealed seams should resist being pulled apart during cook-in. Additonally, it is desirable that the packaging film be substantially comformable to the contained food product. Preferably, this substantial conformability is achieved by the film being heat shrinkable under these conditions so as to form a tightly fitting package, i.e., the film should be heat shrinkable under these time-temperature conditions and should possess sufficient shrink energy so that the packaging film will shrink snugly around the contained product, and especially up to about 55% monoaxial and/or biaxial shrinkage.
The film may be used in the form of seamed or seamless casings or bags. The film is typically of polymeric, thermoplastic construction, either mono layer or multilayer. Commercial examples include CN 510 and CN 610 cook-in casings available from the Cryovac Division of W. R. Grace & Co.-Conn. Cook-in packaging materials are described in U.S. Pat. No. 4,469,742 (Oberle et al); U.S. Pat. No. 4,448,792 (Schirmer); U.S. Pat. No. 4,606,922 (Schirmer); U.S. Pat. No. 4,762,748 (Oberle); and U.S. Pat. No. 4,855,183 (Oberle).
As described in these patents, conventional shrinkable cook-in packages are used to prepare meat products such as bologna, sausage, and mortadella in casings or bags. The package is sometimes placed horizontally or in external molds to control the final shape of the package.
Vertically hung casings are submerged in hot water to prevent the product from distorting unacceptably under its own weight. The hot water cooks the meat product, and in addition provides hydrostatic support for the filled casing during the cooking cycle. This latter feature is important, because the cook-in package can sometimes be over 40 centimeters long and weigh over 3 kilograms. Thus, the hydrostatic forces exerted by the product on the casing can be quite large.
Casings that are shrinkable offer many advantages. One is a reduction in wrinkles formed at the clipped or sealed longitudinal ends of the package. These wrinkles result in product yield loss, which is an economic loss to the producer and packager of products such as smoked and processed meats.
Additionally, shrinkage results in a tighter package with a better appearance following cooking, especially when the casing is not fully filled prior to cooking.
Finally, shrinkable materials also tend to have better optics and oxygen barrier properties as the result of the orientation process used to supply the casing or film with shrink properties.
It would be desirable to use hot air instead of hot water as the cooking medium in these kinds of applications. This is because of the high equipment costs associated with hot water, in-mold and horizontal cook-in systems. Hot air cooking would also be especially desirable in low volume, specialty product applications.
However, if hot air were used instead of hot water in the vertical cook-in process, the advantages of shrink casings can not currently be obtained when cook-in packages are vertically hung, because current materials will distort excessively under the load of the product during the cooking cycle. This distortion typically increases with increasing package weight. "Gourd" shaped packages would be produced, which would be undesirable and potentially unfit for commercial distribution.
The inventor has found that the hydrostatic and pneumatic forces inside the package must be offset to provide good package shape after the cook-in cycle. The benefit of hydrostatic support provided by the hot water would be lost if hot air were used instead. Conventional shrinkable cook-in films have inadequate (too low) shrink force to offset this loss of support. However, increasing the shrink force of the film to an excessively high level can also affect package shape in a different way. If shrink forces are too high, undesirable rounding of the final package can occur.
By making a cook-in material that has a sustained shrink force within a defined range, the advantages of a shrinkable material can be obtained, while avoiding the package distortion that would occur if conventional shrinkable cook-in materials were used in vertically hung packages, and then heated in hot air instead of hot water.
It is an object of the invention to provide a shrinkable cook-in film which exhibits a sustained shrink force within a defined range, in order to provide a cook-in package with acceptable shape when hot air is used as the cooking medium in a vertically hung cooking system.